We used to have a notice, and OH once did slam the door in someone's face in exasperation after he wouldn't leave - the caller responded by kicking over a stone birdbath in the front garden on his way out and smashing it to smithereens. Older and wiser now, we have simply disconnected the doorbell.
We have a large red & white sign on our gate "Beware of the Dog, He Bites, You have been Warned" it's quite a good deterrent, we put it their because of children teasing the dog, he could bite when he gets annoyed, so fair warning but we have noticed 'traders' think twice before entering